This invention relates generally to a method and apparatus for measuring fluid flow and more particularly to a thermal anemometer for measuring the local fluid flow over a thermocouple sensor probe.
Fluid velocity measuring devices are well known and typically utilize thermal anemometer probes inserted into a fluid flow stream. The devices apply a variety of electrical heating and thermal sensing principles, such as electrical resistance heating and sensing or Petier heating and/or cooling with Seebeck effect sensing. The heating or thermal sensing can be effected in a steady state, continuous or cyclic manner.
More specifically cyclic or pulsed anemometers utilize either electric resistance for pulsed heating and dynamic temperature sensing as described in U.S. Pat. Nos. 3,719,083, 3,587,318, 3,905.230, 3,498,128, 4,501,145, or Peltier heating and/or cooling described in U.S. Pat. No. 3,030,806, or Seebeck effect temperature sensing as described in U.S. Pat. Nos. 3,030,806, 3,564,916.
The prior methodology, whether pulsed or constant, generally utilizes fixed probe reference points, such as applying continuous power to the probe and measuring the resistance change due to the cooling from the fluid flow stream. The probe can also be utilized to measure the power necessary to maintain a constant resistance in the fluid flow stream. The devices typically require multiple, complex and fragile sensing elements combined with specialized and dedicated electronic circuitry to perform the measurements.
For example, devices that utilize Seebeck effect temperature sensing generally utilize a tungsten wire probe with a thermocouple connected to it. The wire is utilized as a probe in the fluid flow and can oxidize or deform which changes the characteristics of the probe. The prior methodology requires the characteristics of the probe to be unchanged and hence is very sensitive to changes in the probe caused by oxidation or deformation. This generally eliminates utilization in high temperature or corrosive fluid flow streams.
It would be desirable to measure fluid flow velocity with a simple, durable probe, substantially independent of the probe characteristics in any type of fluid flow stream with a methodology which also can be utilized substantially independent of specialized and dedicated electronic circuitry.